kmspruill242

So I broke down and took the car to a carb guy. Big into drag racing but knows his stuff even with road course cars. So he said with the large amount of vacuum I'm pulling, 17 to 18 inches at idle. I might have been pulling the fuel particles out of suspension. Either way he got the car to idle without making your eyes water.

Again motor is a 2004+ LS6. Cam is factory 550/550 117.5 with 207/218 @ .50.

With the carb he put:
Front,
Jets- 71
Power Valve- 9.5
Air bleeds- 73
Rear,
Jets- 79
Power valve- blocked
Air bleeds- 73

So the car idles great but has a horrible tip in stumble and leans out when you put the motor under load. I only got to drive the car in the parking lot a bit. He doesn't have a dyno. I have access to a dyno but still have to pay for the time. Just wondering if you guys had any suggestions to get me close before I start paying for time. Thanks.

Orangemagic

iTrader: (2)

What kind of carb?

Pop N Wood

iTrader: (1)

What ignition timing are you running and are you using a MAP Sensor? What intake manifold?

kmspruill242

No map sensor. Vacuum taken from hooking up a gauge to the carb. Carb is a Pro-Systems Holley 780. Ignition timing is MSD pill #5 = 15*cranking, 30* @ 800, 32* @ 2000, 34* @ 4200, ramped to 36* @ 6000 all the way out. Intake is a Victor Jr.

bad67ls

iTrader: (2)

um... how can you have that timing with pill 5?? it doesnt even go that high (and if you have an ls6, 36 is probably way too much timing)

timing should be all in by 2200 or so if you ask me as well

kmspruill242

The timing curve is what it says with the paper that's included with the MSD box. I know I can get it better actually hooking it up to a laptop. But I have a MAC so I need to get the serial port to usb adapter.

Orangemagic

iTrader: (2)

How does it seem after first tip in? lean,rich or does it run? Timing at 36 is gonna be a little high but don't think thats your stumble. Try setting the timing at 27 at 2500 then ramp up to 29 at 7k.
Don't know if the carb is new or used but make sure your accel pumps are working. Do you have idle,interm,high speed air bleeds or just high and idle?

Pop N Wood

iTrader: (1)

Speedtigger had a lot of similar issues with his pro systems carb. IIRC that pro system guy just likes to load up the low end.

I have an LS2 (same cam and heads) with a Holley 750 DP and GMPP single plane intake, so I would imagine you could copy my set up and then tell me how your carb guy improves it.

Couple of things. I had to drop my initial timing down below 15°, otherwise my car idled so fast that I had to practically shut the butterflies. This caused a momentary halt every time I came off idle. I run a MAP with I think 10° low vac advance, similar to the #5 pill. I run 35° max timing but like said above that probably isn't an issue with the stumble.

Your jets are real close to mine (71f-74r). I have a 6.5 power valve. So maybe yours is coming in too soon.

Another thing you might try doing is going down a size or two on the primary idle air bleeds. That made a huge difference in my car in the off idle stumble.

How did you set your idle screws? I cheated a bit and have them running just slightly rich.

Last you didn't mention accel pumps. I have a #31 front and #35 rear, using a pink cam in screw hole #1 on both. Make sure there is absolutely no slop between the accel cams and the pump arms. Even the slightest throttle movement should cause a strong squirt down the throat.

kmspruill242

I just have high and idle air bleeds. If I ease into it, it'll fall on it's face and lean out hard.

I have higher rear jets cause he blocked off the rear power valve. He's telling me I should go to a 10.5 in the front. I thought when the power valve came in it richened things up?

Pop N Wood, what vacuum are you pulling at idle? You have the same cam and heads with a little smaller bore but it should be around the same. I'm not too sure where the idle screws are but after the car warms up it'll sit at 13.8 afr all day long. But before it's warm it's sitting around 16.5 afr. I have to baby sit the idle before it warms up.

With the little bit of driving I could do in the parking lot. Mind you I couldn't get into the secondary circuit, but hard into the primary would lean out.

I have a dyno day setup for June 6th and I just want to make sure I can give the tuner a little bit of direction. I've bought two jet sets. One is 70 to 80 and the other is 80 to 90. I've held off on buying the power valve and any air bleeds. Suggestions?

Again thanks guys...

Pop N Wood

iTrader: (1)

I take it you have some type of AFR meter? does it go lean with rapid throttle changes or when slowly increasing the RPM? IMO the first thing you do is adjust jets to get the AFR right with steady throttle settings. Once that is right adjust the accel pump to get rid of bogs on quick throttle movements.

My LS2 has a 4 inch bore, so slightly larger than the LS6, but like you said we should be pretty close to the same set up. The motor pulls a lot of vac at idle, I forget what it measured. The carb has a choke, but no matter all tuning should be done with the motor fully warmed up. Most mechanical secondary carbs don't have a power valve in the secondary.

The idle screws are just over a half turn out. They all need to be the same setting. AFR's run mid 14's at idle. Under cruise they sit at high 14's. Get into the throttle and they come down into the 13's to accel. That is how I know the 6.5 PV is right for my car.

I don't know about you but all this stuff is trial and error for me. Just keep good notes and back out any changes that don't seem to work.

speedtigger

iTrader: (16)

Yep. That Pro Systems carb wants to idle fat and goes lean at tip in. I sent it back 3 times and have countless hours into it.

So, I just upgraded my old 650 double pumper with the holley 750 HP main body and that carb runs perfect. I have not decided if I want to continue to fight with the Pro-Systems carb or just sell it.

kmspruill242

That's what mine was doing at first. It would be diesel rich at idle but with any throttle the AFRs would come up. I just couldn't drive the car (not street legal) to find out if under load it was doing any better. I do have an AFR gauge on the car.